1. Field of the Invention
The present invention relates to an ink jet printer. More particularly, the invention relates to an ink jet recording head used for the bubble jet printer that utilizes bubbling phenomenon, and the like, and to a non-linear electrical element as well.
2. Related Background Art
As regards the technology of non-linear current voltage element, there has been proposed since a long time ago the PTC thermistor having the non-linear characteristic in which the resistance value rises enormously at a certain temperature (curie temperature), and this thermistor has been utilized for various products. For example, there has been proposed in the specification of Japanese Patent Laid-Open Application No. 05-47457 an organic face heat-generating element having the positive temperature coefficient (PTC) characteristic. Also, in the specification of Japanese Patent Laid-Open Application No. 05-258840, there has been proposed a PTC heat-generating device provided with plural PTC elements connected in parallel. Also, in the specification of Japanese Patent Laid-Open Application No. 04-97927, there has been disclosed an ink discharge apparatus that holds the temperature of ink within a desired range of temperature using PTC thermistor heat generating element.
Also, regarding the technology of the non-linear current voltage element, there have been proposed in the specifications of Japanese Patent Laid-Open Application Nos. 2001-71499, 2002-046274, 2002-046275, 2002-067325, and 2002-067326 the applications of MIM element to the bubble jet recording head having the current voltage characteristic (the so-called MIM type current voltage characteristics) in which almost no current runs at a certain voltage or less, and current is allowed to run at a certain voltage or more.
FIG. 7 is a conceptual view that shows the MIM type electrical characteristic. Here, it is desirable to make the current voltage characteristic of the non-linear element the one in which only a sufficiently small current is allowed to run by the application of voltage of a small absolute value on either sides of the positive voltage and negative voltage so that the non-linear element does not generate heat even by the application of non-selective voltage having unsettled polarity. Therefore, as shown in FIG. 7, it is particularly desirable for the current voltage characteristic of non-linear element to set the ratio (V1/V2) at a value of 0.5 to 2.0 for the absolute values between the +V1 and xe2x88x92V2 of the applied voltage that gives current of the absolute value I0, which is equivalent to the current running at the time of voltage application that enables a desired bubbling to be generated. Then, it is also desirable to make the absolute value I0/10 or less for the current that runs when the voltage of +V1/2 and xe2x88x92V2/2 is applied.
On the other hand, regarding the technology of the ink jet recording head, it is generally practiced to provide a recording head, which is applicable to a bubble jet recording method, with fine discharge ports for discharging liquid, flow path to conduct liquid to each discharge port, and heat generating means arranged for a part of each flow path. The bubble jet recording method is a recording method, in which liquid is heated locally in the flow path to a high temperature by use of heat generating means so as to bubble it to generate bubble, and then, by the utilization of such high pressure exerted at the time of bubbling, liquid is pushed out from each of the fine discharge ports so as to enable it to adhere to a recording sheet or the like for recording.
In order to make images highly precise, which are recorded by the recording technology of the kind, it is necessary to discharge extremely fine liquid droplets from the discharge ports arranged in high density. Fundamentally, therefore, it is important to form minute flow paths and minute heat generating means as well. Then, for the bubble jet recording method, a method has been proposed for forming a recording head by using photolithographic process freely, while taking advantage of the simplicity of the structure thereof, so as to arrange discharge ports, flow paths, and heat generating elements in high density for the head (see the specification of Japanese Patent Laid-Open Application No. 08-15629, for example). Also, the discharge amount of liquid droplets should be adjusted in order to discharge fine liquid droplets. To this end, a proposal has been made to use the heat-generating element the amount of heat generation of which is larger in the central portion than the edge portions thereof (see the specification of Japanese Patent Laid-Open Application No. 62-201254).
As heat generating means, it is usually practiced to use the resistive heat-generating element, which is formed by a thin film of tantalum nitride formed in a thickness of approximately 0.05 xcexcm. When this element is energized, joule heat is generated to bubble liquid. On such resistive heat-generating element, there is arranged a cavitation-proof layer formed by metal, such as Ta, in a thickness of approximately 0.2 xcexcm usually though an insulator, such as Sin, formed in a thickness of approximately 0.8 xcexcm in order to prevent the surface of the resistive heat-generating element from being damaged by cavitation.
For the aforesaid recording head that adopts the bubble jet recording method, the resistive heat-generating element for use of ink bubbling usually presents variations to a certain extent due to the resistance of its own finish and the resistance generated by the connected wiring. As a result, even if voltage is applied under constant condition, there occur variations in the voltage drop due to resistance, and the heating amount of the heater formed by such resistive heat-generating element tends to vary accordingly. Therefore, for the reasons that any influences that may be given to the inferior image quality due to such variation of heating amount should be avoided, among some others, the driving voltage, which is required for driving the heater array formed by plural heaters, is provided with a higher voltage value than the voltage value needed usually for the stabilized bubbling on the entire surface of each individual resistive heat-generating element, which faces liquid. Such voltage value is particularly set at as much as approximately 1.2 times the required voltage value.
However, when the driving voltage is set at a higher value as described above, an excessive voltage is applied to the average heater beyond the voltage needed for the bubbling on the entire surface. As a result, there encountered a problem, in principle, that unwanted heating continues even after bubbling.
More specifically, if a heater is driven with 1 xcexcs-pulse, for example, it is typical that bubbling takes place at approximately 6 xcexcs, and than, unwanted heating (excessive heating) by the heater continues after bubbling, thus causing the heater surface to reach typically at a temperature of as high as 600 to 700xc2x0 C. approximately against the bubbling temperature of 300xc2x0 C. approximately. Then, depending on conditions, there is a problem that such temperature is made higher still.
To described this problem further in detail, due to the principle of the continuation of excessive heating described above, there is a fire that the following problems are encountered:
(1) In terms of effective utilization of energy, it is not preferable to supply energy wastefully after bubbling.
(2) It is necessary to design the heat resistance of the heater material at a value higher than needed in consideration of the principle cause to make the heater temperature excessively high.
Also, depending on cases, this may bring about thermal destruction, and the deterioration of durability by the repeated abrupt change of temperatures.
Under the circumstances, if it is possible to materialize the heater for bubble jet use, which is capable of suppressing the excessive heating after bubbling, there is a possibility to be able to provide a head for bubble jet use that should be preferable from the viewpoint of energy saving and enhanced durability, and the prevention of thermal destruction as well.
Meanwhile, it is a prerequisite for many of the conventional heads that heat generating elements, diodes, and logic circuitry are incorporated on a silicon base plate simultaneously by means of semiconductor process (such method as ion implantation). As a result, the head the number of nozzles of which is comparatively small can be formed compactly with an advantage that it can be manufactured in a single process. However, in a case of a multiple head that has a full length to cover the width of a recording sheet, a length of approximately 305 mm is needed if it is formed integrally. This makes it difficult to use usual silicon wafer, and there is a fear that the method of manufacture that should be adopted therefor becomes highly costly.
Therefore, if it is possible to perform the matrix drive of the heat generating element for bubbling use by use of the MIM element that can be produced without depending on the conventional semiconductor process, such as ion implantation, there is a possibility to provide an elongated ink jet head at low costs.
Also, for the resistive heat-generating element of the heater portion of a recording head for use of bubble jet, it is necessary to supply a power having the density of approximately 0.1 GM/m2 or more to the resistive element serially connected with MIM element or to the MIM element itself. Then, there is a fear to invite the destruction of the MIM element itself by the large electrical current. The power loss caused by such MIM element itself is largely small in the conventional MIM-application products, such as a liquid crystal display. There has been no problem at all. In other words, the problem encountered here is conceivably the one related particularly to the MIM element for the bubble jet use that deals with a large power.
Here, in particular, there is a fear for the conventional MIM element that electrical current is concentrated on the portion having a narrower gap between electrodes if the distance between electrodes varies, which makes uniform heat generating difficult.
FIG. 8 is a view that shows one example of the temporal changes of temperature distribution by the MIM element that has in-plane variations with respect to the gap between electrodes or the like. For the MIM element, if there exists such in-plane variation with respect to the gap between electrodes or the like, electrical current concentrates on the narrower portion of the electrode gap at first. As a result, the uneven distribution of temperature takes place at the outset. Then, in continuation, the resistance value of the high-temperature portion is made lower due to the NTC (negative temperature coefficient) of the resistance value of the tunnel current. As a result, the high-temperature portion is in a state of having higher temperature, thus leading to the destruction. In this respect, as the electrical conduction mechanism in the insulator of the MIM element, there have been known an electrical conduction of hopping type where plural tunnel actions are repeated in a conductor such as the conduction of Poole-Frenkel type, a comparatively simple tunnel conduction such as the conduction of Fowler-Nordhaim type, and the like.
Also, for the MIM element, if the concentration of electrical current occurs as described earlier, the resistance of the portion where the current concentration takes place is made lower still and the temperature rises further due to the NTC (negative temperature coefficient) characteristic of the resistance value of the MIM element resulting from the tunnel current.
Now, therefore, the present invention is designed in consideration of the problems discussed above. It is an object of the invention to provide a non-linear electrical element having the electrical characteristic of MIM type that functions protect the element automatically from unwanted temperature rise.
Also, it is another object of the invention to provide a highly durable, energy saving ink jet recording head, which is capable of providing an elongated head at lower costs.
In order to achieve the aforesaid objects, the ink jet recording head of the present invention is an ink jet recording head for discharging liquid by bubbling liquid with heat generating means. This heat generating means is provided with a laminated member having a pair of electrodes, a resistance layer having positive resistance temperature coefficient that raises resistance value abruptly when temperature rises higher than a predetermined temperature, and an insulation layer for enabling electrical current to run by the application of voltage higher than a predetermined voltage. Here, the pair of electrodes sandwich the resistance layer having the positive resistance temperature coefficient, and the insulation layer.
Also, it may be possible to arrange the thickness of the insulation layer of the ink jet recording head of the invention to be 4 nm or more and 40 nm or less.
Also, for the ink jet recording head of the invention, the temperature, which causes the resistance value of the resistance layer having the positive resistance temperature coefficient to rise abruptly, may be arranged to be near the bubbling temperature of liquid or 250xc2x0 C. or more and 490xc2x0 C. or less.
Further, the ink jet recording head of the invention may be the one in which the heat generating means is formed essentially by serially connecting MIM element and PTC thermistor, and the insulation layer does not allow electrical current to run even by the application of voltage lower than a predetermined voltage, and allows electrical current to run by the application of voltage higher than the predetermined voltage, and then, the resistance layer cuts electrical current after bubbling of liquid.
Also, the ink jet recording head of the invention may be the one which comprises a pair of electrodes; a resistance layer having the positive resistance temperature coefficient raising resistance value abruptly when temperature rises higher than a predetermined temperature; an insulation layer allowing electrical current to run by the application of voltage higher than a predetermined voltage; and an insulating member having a contact hole formed therefor. For this ink jet recording head, the pair of electrodes sandwich the resistance layer having the positive resistance temperature coefficient, and the insulation layer in the contact hole.
The non-linear electrical element of the invention is a non-linear electrical element having non-linear resistive characteristic, which comprises
a pair of electrodes; a resistance layer having the positive resistance temperature coefficient raising resistance value abruptly when temperature rises higher than a predetermined temperature; and an insulation layer allowing electrical current to run by the application of voltage higher than a predetermined voltage. For this non-linear electrical element, the pair of electrodes sandwich the resistance layer having the positive resistance temperature coefficient, and the insulation layer.
Also, for the non-linear electrical element of the invention, it may be possible to arrange the thickness of the insulation layer to be 4 nm or more and 40 nm or less or to make it essentially a laminated structure formed by a serial circuit of MIM element and PTC thermistor.
Further, the non-linear electrical element of the invention may be the one, which comprises a pair of electrodes; a resistive heat generating element sandwiched between the pair of electrodes, having the positive resistance temperature coefficient raising resistance value abruptly when temperature rises higher than a predetermined temperature; an electrical insulating element covered by the resistive heat generating element, having a contact hole formed therefor to enable one electrode of the pair of electrodes to be electrically connected with the resistive heat generating element; and an insulation layer covering one of the electrodes in the contact hole to allow electrical current to run by the application of voltage higher than a predetermined voltage.
The ink jet recording head of the present invention described above is provided with a pair of electrodes, a resistance layer having positive resistance temperature coefficient that raises resistance value abruptly when temperature rises higher than a predetermined temperature, and an insulation layer for enabling electrical current to run by the application of voltage higher than a predetermined voltage. Then, the laminated element, in which the pair of electrodes sandwich the resistance layer having the positive resistance temperature coefficient, and the insulation layer, constitutes heat-generating means. In other words, the heat-generating means of the ink jet recording head of the present invention forms essentially a serial circuit formed by MIM element and PTC thermistor. Thus, electrical current does not run by the application of voltage lower than a predetermined voltage, but it is allowed to run when voltage higher than the predetermined voltage is applied. FIG. 9 is a diagram that shows the heat-generating means of the ink jet recording head of the present invention as an equivalent circuit of MIM element 101 and PTC thermistor 100.
As indicated in the matrix circuit diagram shown in FIG. 10, it is possible to form the matrix circuit capable of performing matrix driving for the ink jet recording head of bubble jet type by means of the serial circuit of the MIM element 101 and the PTC thermistor 100 for. In other words, with the arrangement of heat-generating means on each intersecting point of the wiring in the column direction X1, X2, . . . and the line direction Y1, Y2, . . . , it is made possible to enable electrical current to run by the application of voltage higher than the voltage existing across a pair of electrodes for generating heat, and then, heat-generating means heats liquid for bubbling, and then, after bubbling, electrical current is automatically cut by means of the PTC thermistor even in a state of voltage being applied. Here, when the applied voltage is lower than the voltage existing across the pair of electrodes, electrical current does not run, thus generating no heat. In other words, the ink jet recording head of the present invention makes it possible to automatically cut electrical current even in a state of voltage being applied for the prevention of excessive heat generation. Therefore, at the same time that the excessive energy consumption is suppressed, heat-generating means is prevented from being damaged so as to enhance the durability of heat-generating means.
Also, with the arrangement of the non-linear element that presents MIM type current voltage characteristics on each intersecting point of the matrix electrodes as described above, unwanted heat generation by the bias voltage at the time of matrix driving, which takes place on the points yet to be selected, can be suppressed, hence making it possible to perform the matrix driving of heaters. Also, by the matrix driving, it becomes easer to separate driver and heater, which produces an effect that makes a large-scale production possible by use of inexpensive non-Si base plate.
Also, the non-linear electrical element of the present invention described above is provided with the resistive heat-generating element having the positive resistance temperature coefficient that raises resistance value abruptly when temperature rises more than a predetermined temperature, and the electrical insulating layer that covers the resistive heat-generating element, thus making it possible to suppress electrical current running to the current concentrated portion by enabling the specific resistance of the resistance layer to rise abruptly, while maintaining the characteristics of the MIM element, even if the current concentration should take place to raise the element temperature locally. In this manner, it is possible to stably deal with the large current enormously great in the turned-on condition, which is the feature of the MIM type current voltage characteristics.
Also, with the thickness of the insulation layer of 4 nm or more and 40 nm or less, it is made possible to give preferable MIM type electrical characteristics to the matrix driving of the liquid discharge unit of the bubble jet type.
As the electric conduction mechanism of the insulating member of the MIM element, there has been known the hopping type electric conduction where plural tunneling actions are repeated in the insulating member, such as Poole-Frenkel type conduction or a comparatively simple tunneling conduction such as Fowler-Nordhaim type conduction, among some others.
In order to allow electrical current of tunnel type to flow so that the electrical current runs on the junction element, there is a need for making the gap between electrodes extremely narrow. The limit of the film thickness of the insulating member where electrical current flows to the MIM element or the limit of electrode gap depends largely on the kinds of the insulating material and electrode material, and the conduction mechanism as well. However, it is desirable to make the electrode gap 100 nm or less, for example, in order to enable the useful electrical current to run as the MIM element. Further, it is preferable to make the electrode gap 40 nm or less in order to obtain the large current, which is needed to drive the bubble jet recording head, at low voltage. Also, there is a fear that ion on the metallic surface of electrodes generates electric field radiation if the electrode gap is too narrow. Therefore, it is desirable to make the electrode gap 1 nm or more. Further, in order to obtain the tunnel junction that generates stable tunnel conduction, the electrode gap should be made 4 nm or more. In other words, it is particularly preferable to use the MIM element having the electrode distance of 1 nm or more and 100 nm or less, or more preferably, 4 nm or more and 40 nm or less, as the non-linear element.
Also, with the arrangement to make the temperature, at which the resistance layer having the aforesaid positive resistance temperature coefficient is raised abruptly, to be near the bubbling temperature of liquid, electrical current is automatically cut immediately after bubbling. Also, the temperature of the kind should preferably be 250xc2x0 C. or more and 490xc2x0 C. or less in consideration of the tendency that the bubbling temperature of ink in general, and the surface temperature of the heat-generating element, which is in contact with ink, are made lower than the inner temperature of the heat-generating element.
As has been described, in accordance with the present invention, it is possible to provide the non-linear electrical element having MIM type electrical characteristics, which is provided with the function to protect the element automatically from the unwanted temperature rise, while providing an energy-saving, highly durable elongated ink jet recording head at low costs.